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couple questions regarding li-poly batteries

Hi - I'm working on a circuit that needs a minimum of 8VDC in. I'm planning on using 3 of these li-poly batteries in series to power it:

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One thing that wasn't mentioned in the datasheet is how the voltage drops as the battery is discharged. It says that the 'working voltage' is 3.7V, so does that mean the battery is dead by the time it drops below 3.7V? Anyways - can anybody tell me if 3 of these is enough? Or should I step it up to 4?

Also - does anybody know of any other similarly rated batteries? (other manufacturers/types)

So far these batteries have the highest watt hour / gram ratio of any batteries I've looked at. (even much better than li-polys from other manufacturers that charge more than twice as much. Strange...)

On a related note - how difficult is charging a li-poly battery? This is for a one-off board, so I was planning on just buying a li-poly charger, but if it isn't too difficult (maybe some pre-made chips for the purpose?) it sure would be neat to be able to connect a DC power supply to the board and have it charge there.

Thanks in advance,

-M. Noone

Reply to
M. Noone
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Reply to
M. Noone

I became very aware of the dangers of li-polys when an acquaintance of mine burned down his shed last summer, taking with it thousands and thousands of dollars in RC equipment. The lesson I got from that is to never charge li-polys on the wrong setting, and never leave them unattended.

2C is fairly low compared to some of the hobby batteries - but luckily my needs are such that high current is not necessary, though high capacity and low weigtht are.

Single cell charging chips are not problematic as the three cells will all be connected in series on the board itself - so it would be easy enough to have 3 seperate charger chips - one for each cell. Any reccomendations for suitable ICs?

Thanks,

-M. Noone

Reply to
M. Noone

LiPos tend to go boom when mistreated, hence the dire warnings in the info sheet. They're quite common in R/C hobbies, due mainly to the very favorable energy density. 2C is pretty low for continuous discharge; hobby grade batteries are rating themselves at 10C and higher these days. For whatever that's worth.

Lithium Ion charger chips are pretty common. They're used in cellphones and PDA's, so 1 and 2 cell chargers are quite common. More than 3 cells is hard to find.

You'll have to contact the manufacturer or distributor for that particular battery's discharge curve. Or just buy one and graph it yourself. In general, peak voltage is 4.1 volts after a charge, drops quickly to 3.7 or so, and holds relatively flat until near charge capacity, where it falls off sharply. The shape depends very much on the discharge rate. Lipo's should not be drawn down much below 3 volts. I believe the hard cut-off is around

2.7 volts; the cells reverse charge at that point and are permanently damaged.

Reply to
Mike Young

The discharge profiles of Li-Ion and Li-Polymer are very similar. From a commercial design I did recently for a client, the testing curves show a significant drop initially from the OC voltage of 4v2 to about 3v75. The curve is then fairly linear down to about 3v3 or so, and starts to droop progressively down to the 3v0 cutoff we employed.

I have emailed a copy of the Excel chart for two series cells to your gmail address - I hope it was valid.

Reply to
budgie

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